LibWeb: Add a bare-bones HTML event loop with a task queue

This patch attaches a HTML::EventLoop to the main thread JS::VM used
for JavaScript bindings in the web engine.

The goal here is to model the various task scheduling mechanisms of the
HTML specification.

Userland/Libraries/LibWeb/HTML/EventLoop/TaskQueue.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Queue.h>
+#include <LibWeb/HTML/EventLoop/Task.h>
+
+namespace Web::HTML {
+
+class TaskQueue {
+public:
+    TaskQueue();
+    ~TaskQueue();
+
+    bool is_empty() const { return m_tasks.is_empty(); }
+
+    void add(NonnullOwnPtr<HTML::Task>);
+    OwnPtr<HTML::Task> take_first_runnable() { return m_tasks.dequeue(); }
+
+private:
+    Queue<NonnullOwnPtr<HTML::Task>> m_tasks;
+};
+
+}